1. Field of the Invention
This invention relates generally to desalinating processes, and more particularly to a process for desalinating seawater or brine or purifying fresh containing minerals, salts, and other dissolved solids while simultaneously generating power. Efficiency is improved by utilizing a low pressure boiler at lower temperatures for desalination in conjunction with a high pressure boiler for producing power.
2. Brief Description of the Prior Art
As world population increases, demand for fresh water and power will also increase. Pollutants and drought result in a shortage of fresh water in many locations. Therefore, it would be desirable to provide a process utilizing desalination and distillation combined with power generation whereby demand for fresh water and power can be simultaneously satisfied.
Most previous methods of desalination have been stand-alone processes. Hence, they have focused upon energy efficiency to satisfy economics. Several of the commercial methods include reverse osmosis, evaporation, and vapor recompression. Dual purpose power plants have also been utilized.
Reverse osmosis is a technology wherein fresh water is extracted from saline water by pressure. This is accomplished by circulating saline water under high pressure (i.e., 1000-2000 psig) around a loop. One portion of the loop is adjacent to a membrane. The membrane selectively allows water to pass through it while preventing the passage of most ions. Effectively, fresh water is squeezed from the saline water. Excellent energy efficiency can be achieved by this method. However, the membranes are prone to pluggage and in practice the fresh water produced is not completely free of dissolved salts. The present process, on the other hand, produces fresh water by a phase change and produces power.
Evaporation is the boiling of salinous water by the addition of heat followed by the condensation of the steam by heat exchange. Evaporators may be classified as boiling or flashing. No work is performed by the system and a large amount of energy input is required. This method is the least energy efficient of the existing methods. The present process, on the other hand, performs work and partial condensation of the steam may be accomplished by doing the work.
Vapor recompression is a technology wherein water boils itself. This is accomplished by boiling water at low pressure to produce water vapor. The water vapor is compressed and heated by doing work upon it. The heated water vapor is then condensed by heat exchange against the boiling water. The net result is that a phase change is accomplished by doing work. The energy efficiency of the system is controlled by the amount of heating of the water vapor. Small temperature increases result in high energy efficiencies and hence low operating costs for energy. Unfortunately, small temperature increases also result in large amounts of heat exchange area and hence high capital outlays. The present process, on the other hand, requires less heat exchanger area for a given duty and condensation may be at least partially achieved by doing work. With the present system, work is withdrawn from the system rather than input into the system.
Dual purpose desalination/power plants currently in use produce fresh water by using the exhaust steam as a source of heat for an evaporator. The exhaust steam is condensed against the boiler of the evaporator. As the boiler duty increases with fresh water production, the temperature of the condensing exhaust steam also increases. This reduces the thermodynamic efficiency of the power plant providing the steam. In the present process efficiency of the power plant is not adversely affected by increasing the fresh water production rate.
Power generation using steam expansion is a common process. Condensate is fed to a boiler and heated. Steam is removed from the boiler and typically superheated. It then expands across a turbine, thereby doing work. The steam is then condensed and recycled to the boiler. A moderate amount of liquid is intermittently withdrawn from the boiler to prevent sludge accumulation. Treated fresh water is added to the system to compensate for material losses. The present process, on the other hand, withdraws the condensate as a product. Also, treated salinous water is fed to the boiler and liquid is continuously removed from the boiler to reduce scaling and prevent supersaturation by salt. In addition, the steam produced is washed by a stream of condensate to remove volatized salts and other inorganic compounds such as silica. Efficiency is further improved by utilizing a low pressure boiler at lower temperatures for desalination in conjunction with a high pressure boiler for producing power.
There are several patents which disclose various desalinating processes, some of which also generate power.
Ellis et al, U.S. Patent discloses a process which utilizes geothermal brine to generate power in a closed system with the exclusion of air to minimize corrosion. Steam from geothermal brine contains significant quantities of soluble salts including sodium and potassium chloride, calcium salts and iron and manganese salts, which have a strong corrosive action on turbine blades and related equipment. In this process, hot geothermal brine is flashed in a flash zone to form steam and concentrated brine and the steam is used to drive a power-generating turbine. The exhaust steam from the turbine is condensed and the major portion of the condensed steam is combined with the concentrated brine to form a restored brine, and the restored brine is returned to the geothermal hot brine well. There is no suggestion of a fresh water product.
Kutchinson et al, U.S. Pat. No. 3,893,299 discloses a geothermal heat recovery process wherein hot water from a geothermal well is passed through successive flash chambers operating at successively lower temperatures and the steam from each flash chamber is passed in heat exchange relationship with a working fluid operating in a closed loop which is expanded in a power extracting gas expansion device for generating power. The hot fluid at the output of each heat exchange is either combined with the steam at the output of the next flash chamber or applied to the input of the next flash chamber with the hot fluid that is not converted to steam. There is no suggestion of a fresh water product.
Spears, Jr., U.S. Pat. No. 4,078,976 discloses a potable recovery and power generating process which utilizes solar power for recovering potable water from salinous water. A portion of salinous water and an air stream are introduced into a solar radiation heat sink and heated water-containing air is withdrawn and condensed into potable drinking water. The heated salinous water is withdrawn from the solar radiation heat sink and recycled, and a part of the heated salinous water is flashed and the resultant vapor is passed through turbines to generate power and the exiting turbine vapors are cooled or condensed by contact with a second portion of the salinous water to recover addition potable water.
Pitcher, U.S. Pat. No. 4,267,022 and Gress, U.S. Pat. No. 4,310,382 disclose processes which utilize air as a working fluid for desalination and heat pumps for transferring latent heat associated with vaporizing or condensing water from one part of the process to another. Both processes require work input rather than producing work.
Mock, U.S. Pat. No. 4,276,124 and Elmore, U.S. Pat. No. 5,096,543 are essentially low-efficiency evaporator systems which utilize air as a working fluid to transport water vapor from one part of the system to another.
Becker, U.S. Pat. No. 3,557,863 discloses a process for obtaining fresh water from saline water by injecting saline water through nozzles into a hot high pressure gas directed into an evaporation chamber to evaporate the saline water and generate a gas-vapor mixture and a precipitate. The gas-vapor mixture and the precipitate are separately withdrawn from the chamber. The gas-vapor mixture is engine expanded and then cooled to condense out fresh water. Becker teaches away from the use of heated metallic heat exchanger surfaces and teaches away from introducing fresh wash water into the steam to wash the steam such that it is substantially free of trace salts, minerals, and dissolved solids, and then expanding washed steam across the turbine.
Williamson, U.S. Pat. No. 3,489,652 teaches indirect contact of the saline water with the heat source in a heat exchanger at the first part of the process and then flash evaporating the saline water in successive stages in a multi-stage flash evaporator to produce a vapor fraction and a brine fraction in each stage and the brine is finally discharged as waste. Williamson does not use a wash column, and the multi-stage evaporation process lowers the temperature of the steam that reaches the turbine which lowers the efficiency of the turbine.
The present invention is distinguished over the prior art in general, and these patents in particular by a process and apparatus for desalinating seawater or brine and purifying water containing minerals, salts, and other dissolved solids while simultaneously generating power. The salinous water is heated in a boiler to form steam and a concentrated brine. The concentrated brine is removed from the boiler, the steam produced in the boiler is washed with fresh water to remove trace salts and inorganic materials, and water bearing trace salts and inorganic materials are returned to the boiler. The washed steam is expanded across a turbine to generate electrical or mechanical power which is utilized as a product. The steam exhausted from the turbine is collected and condensed, and one portion of the condensed water is utilized as a fresh water product and another portion of the condensed water is used as the wash water to wash the steam produced in the boiler. Energy efficiency is improved by heat exchanging the hot concentrated brine against the salinous feed water or by flashing the brine to produce steam. Boiler scaling and corrosion may be controlled by feed water pretreatment. By utilizing distillation combined with power generation, demand for fresh water and power can be satisfied simultaneously. Efficiency is further improved by utilizing a low pressure boiler at lower temperatures for desalination in conjunction with a high pressure boiler for producing power.